1. Field of the Invention
The present invention relates to a method of manufacturing a master optical disk, and more particularly to a method of manufacturing a master optical disk wherein high-density cutting is effected by using a solid immersion lens (SIL)
2. Description of the Related Art
Conventionally, a technique is known in which information is recorded on an optical disk by using a solid immersion lens (hereafter, referred to as the SIL).
Here, referring to FIGS. 6A to 6C, a description will be given of the essential steps of the process for manufacturing a master optical disk.
(1) Step of Coating with a Photoresist (see FIG. 6A)
After a glass substrate 50 worked into a predetermined shape is coated with an organic primer such as hexamethyldisilazane (HMDS) so as to enhance adhesion to a photosensitive material (photoresist), the glass substrate 50 is spin-coated with a photoresist 51, as shown in FIG. 6A.
Then, a solvent in the photoresist 51 is allowed to evaporate, and the glass substrate 50 with the photoresist 51 coated thereon is prebaked by using a clean oven or the like so as to stabilize the film of the photoresist 51.
(2) Exposure Step (see FIG. 6B)
A recording signal is supplied to a recording head of a cutting apparatus (not shown) from a external circuit (not shown), recording light is turned on and off by an optical modulator (not shown), and recording light L is introduced to an objective lens 52, as shown in FIG. 7.
At this time, the recording light L which is converged by the objective lens is made incident upon the spherical surface of an SIL 53, and the recording light L is converged substantially on the center of a flat surface, i.e., an emergent surface, of the SIL 53.
In this case, if it is assumed that a super spherical lens is used as the SIL 53, an equivalent numerical aperture NA of the optical system using the SIL becomes EQU NA=n.sup.2.times.NA
where n is the refractive index of the SIL.
Accordingly, if the wavelength of the laser light is assumed to be .lambda., the beam spot diameter L.sub.BEAM of the recording light is determined by
L.sub.BEAM =.lambda./(n.sup.2.times.NA)
More specifically, the recording light L converged by the objective lens 52 is made incident upon the SIL 53 disposed spaced apart by a gap (see FIG. 7) from the surface of the photoresist 51, is converged by the SIL 53, and is applied to the photoresist 51 on the glass substrate 50, thereby cutting (recording) information spirally.
At this time, the beam diameter at the emergent surface of the SIL 53 becomes .phi.D1 as shown in the detailed explanatory diagram of FIG. 8, and becomes substantially equal to the beam diameter on the photoresist 51 if the gap is set to effect near-field recording, i.e., if the gap is set to a distance less than the 1/4 wavelength of the recording light.
(3) Development Step (see FIG. 6C)
Next, as development process is effected, a master glass disk is formed in which recess-shaped pits are arranged spirally on its surface.
In the case where the above-described conventional cutting apparatus is used, since the SIL 53 effects near-field recording with respect to the photoresist 51 on the glass substrate 50, the recording light L is converged (with a diameter of .phi. D1) inside the SIL 53. Hence, there has been a problem in that the power density in the vicinity of the emergent surface of the SIL 53 becomes high, resulting in the deterioration (so-called tarnish) of the SIL such as clouding.
In addition, dust which is present in the air even in a clean room is generally larger than the gap interval (10 [nm] to 100 [nm]) of the SIL. Hence, there has been a problem in that as the dust enters the gap portion due to static electricity and the like, the floating head is sprung up, making accurate writing impossible until the interval between the SIL and a protective film is restored to its original interval.
In addition, there has been another problem in that dust becomes attached to the substrate of the optical disk due to static electricity, and the SIL, the protective film, or the photoresist is defected by the attached dust.